Transimpedance amplifiers are typically utilized to provide output voltage signals proportional to input current signals. Such amplifiers are normally implemented by providing a feedback resistor across the input and output of a voltage amplifier.
Transimpedance amplifiers are used to detect optical signals in optical transmission systems. In these applications, optical signals which are generally transmitted through an optical fiber, are irradiated onto a PIN photodiode or avalanche photodiode which is connected to the input terminal of the amplifier. The photodiode converts the optical signals into currents which is applied to the input of the amplifier. The amplifier thus provides at its output terminal a voltage proportional to the diode current. In optical transmission systems, the information transmitted is usually digital and generally in the form of a pulse train.
In a transimpedance amplifier, the value of the feedback resistor directly relates to the performance of the amplifier. For example, the sensitivity of the transimpedance amplifier is proportional to the value of the feedback resistor, whereas the bandwidth of the amplifier is inversely proportional to the value of the feedback resistor. Additionally, the output voltage of the amplifier is a product of the input current and the value of the feedback resistor, and is thus proportional to the value of the feedback resistor.
The dependency of the amplifier's performance on the value of the feedback resistor presents special problems in utilizing transimpedance amplifiers to detect optical signals in an optical transmission system. In optical transmission applications, it is desirable that the transimpedance amplifiers are fabricated prior to their installation, and that the same transimpedance amplifiers are used for receiving strong optical signals such as in a Local Area Network where the distance of transmission is short as for receiving weaker optical signals in an optical network where the distance of transmission is long.
Thus, in a transimpedance amplifier utilized for optical signal detection, if the value of the feedback resistor is chosen to be high for high optical sensitivity, the bandwidth of the amplifier will be low due to the high value of the feedback resistor. In addition, when applying such transimpedance amplifier for detecting optical signals having high optical power, the high input optical power generates high input current to the amplifier, which in turn produces an output voltage above the output voltage that the amplifier can supply at its output without incurring significant distortion to the output waveform; Consequently, the output waveform becomes distorted and the clipping of the output pulses occurs. For a typical GaAs amplifier, if the peak to peak output voltage exceeds about 1 volt, the output pulse will begin to clip and the amplifier begins to lose its virtual ground at the input terminal of the amplifier.
On the other hand, if the value of the feedback resistor is selected to be low for high bandwidth and large dynamic range (i.e. being capable to accommodate high input current without resulting in significant distortion to the output waveform), the amplifier will have a low sensitivity which results in high bit error rate at low input current. Additionally, in a transimpedance amplifier, if the value of the feedback resistor is lower than a certain value, the transimpedance amplifier may produce oscillation and become unstable, which sets a lower limit for the value of the feedback resistor.
It is therefor an object of the present invention to provide an automatic transimpedance control amplifier which has a wide bandwidth, high sensitivity, and more importantly, a large dynamic range.